1. Field of the Invention
The present invention relates to electronic testing, more particularly, to fixtures for testing circuit boards and components.
2. Description of the Related Art
Computer designers need to understand what happens to signals as they travel to and from the outside world to the processor. Along that path, the signal will pass through several feet of cable, a variety of board edge connectors, solder joints, inches of board trace, vias, and socket contacts before they finally reach the processor. All of those transitions have unwanted effects, like reflections and noise. If the designer can measure those effects, they can plan for them in the design.
These effects are measured with instrumentation like vector network analyzers and time domain reflectometers. The most widely used interface between those instruments and the board is a coplanar probe. A coplanar probe is constructed as follows. A coaxial cable is terminated on one end with flexible, conductive cantilever beams. One or more of the beams connect to the cable's ground shield and one connects to the cable's center conductor. The beams are spaced to match the cable's impedance environment. The beam end of the cable is held in an armature and the other end of the cable has a standard coaxial connector for instrumentation cables. The armature holds the beams at a shallow angle with respect to the board surface.
A large x-y-z stage is used to move the armature so the beams touch the desired spot on the board. Motion happens first along a plane just above the board surface, then the beams are lowered along the z axis with the appropriate contact force. The user can only probe a lone single-ended or differential pair signal location on the board. If a second signal location needs probing, a second probe and x-y-z stage are needed. The signal locations are small and tightly spaced, meaning the probe motion needs to be monitored under magnification. The cantilever beams have limited travel making them sensitive to disturbances in the board's z-axis position. Often, the boards need to be mounted on a special pneumatic table to dampen building vibrations or unintended bumps from the user.
All the extra hardware needed to probe even one signal location makes the overall solution large and cumbersome. In some cases, the task requires more than one person—one manning the x-y-z stage and the other guiding the motion through a microscope. Setup can take an hour or more per position. Alternative solutions that use automation are very expensive. Furthermore, even after the probe has been positioned, the user still needs to deal with how fragile they are. Bumping the board by accident can snap the beams and cause lengthy delays in the testing schedule.